Schmitt Trigger Circuit Diagram:

Inverting Schmitt Trigger – A Schmitt Trigger Circuit Diagram is a fast-operating voltage level detector. When the input voltage arrives at a level determined by the circuit components, the output voltage switches rapidly between its maximum positive level and its maximum negative level.

An op-amp inverting Schmitt Trigger Circuit Diagram is shown in Fig. 14­-33 together with input and output waveforms. At first glance the circuit looks like a noninverting amplifier. But note that (unlike a noninverting amplifier) the input voltage (V_i) is applied to the inverting input terminal, and the feedback voltage goes to the noninverting input. The waveforms show that the output switches rapidly from the positive saturation (+V_o(sat)) voltage to the negative saturation level (-V_o(sat)) when the input exceeds a certain positive level; the upper trigger point (UTP). Similarly, the output voltage switches from low to high when the input goes below a negative triggering point; the lower trigger point (LTP).

Note that after V_i has increased to the UTP and V_o has switched to -V_o(sat), the output remains at -V_o(sat) even when V_i falls below the UTP. Switch over from -V_o(sat) to +V_o(sat) does not occur until V_i = LTP. Similarly, after V_i has been reduced to the LTP and V_o has switched to +V_o(sat), the output remains at +V_o(sat) when V_i is increased above the LTP. Switch-over from +V_o(sat) to -V_o(sat) does not occur again until V_i = UTP.

Triggering Points:

If the output voltage to the circuit in Fig. 14-33 Is high, the voltage at the noninverting terminal is,

If the input voltage (at the inverting input terminal) is below V_R2 (at the noninverting input), the output voltage is kept at its high positive level. For the output to switch to its low level, the input voltage must exceed V_R2 by a very small amount (approximately 70 μV for a 741 op-amp). So, the UTP essentially equals V_R2.

When the output is negative, the LTP can be calculated as,

Input/Output Characteristic:

A graph of output voltage (V_o) versus input voltage (V_i) can be plotted for an inverting Schmitt Trigger Circuit Diagram, as shown in Fig. 14­-34. This is the circuit input/output characteristic.

To understand the characteristic, consider the voltages at each of the numbered points on the graph:

• At point 1, V_o = +V_o(sat), and V_i < LTP, thus keeping V_o = +V_o(sat).
• From point 1 through points 2 and 3, V_o remains at +V_o(sat) as V_i is increased through the LTP and through zero, until V_i = UTP.
• From point 3 to point 4, V_o switches rapidly from +V_o(sat) to -V_o(sat) when V_i = UTP.
• From point 4 to point 5, V_o remains at -V_o(sat) as V_i is increased above the UTP.
• From point 5 through points 4 and 6, V_o remains at -V_o(sat) as V_i is reduced through the UTP and through zero, until V_i = LTP.
• From point 6 to point 2, V_o switches rapidly from -V_o(sat) to +V_o(sat) when V_i = LTP.
• From point 2 to point 1, V_o remains at +V_o(sat) as V_i is reduced below the LTP.

The difference between the UTP and the LTP is termed hysteresis. Some applications require a small amount of hysteresis, and for other applications a large amount of hysteresis is essential.

Circuit Design:

Design procedure for a Schmitt Trigger Circuit Diagram is similar to op-amp amplifier design. A voltage divider current (I₂ in Fig. 14-33) is selected much larger than the op-amp input bias current. The resistor values are then calculated as,

Adjusting the Trigger Points:

Many Schmitt trigger circuit applications require UTP and LTP levels that are not equal in magnitude. This is usually achieved by the use of diodes, as illustrated in Fig. 14-36.

The circuit shown in Fig. 14-36(a) simply has a diode (D₁) connected in series with resistor R₁. The diode is forward biased only when the op-amp output is a positive quantity. At this time, the UTP is V_R2, as before. When V_o is negative, D₁ is reverse biased, making I₂ equal zero. Consequently, there is no voltage drop across R₂, and so the noninverting terminal is grounded via R₂. This gives a zero level for the LTP. Thus, this circuit has a positive UTP and a zero voltage LTP.

Figure 14-36(b) shows a circuit with two different-level trigger points. When V_o is positive, D₁ is forward biased and D₂ is reversed, and the UTP is set by resistors R₁ and R₂. With V_o negative, D₂ is forward biased and D₁ is reversed. The LTP is now determined by the resistances of R₃ and R₂.

The diode forward voltage drop (V_F) must be accounted for when calculating the trigger points for both of the circuits in Fig. 14-36. This is done simply by replacing V_o with (V_o(sat) – V_F) in Eqs. 14-25 and 14-26. Another important design consideration is that the voltage divider current (I₂) should normally be a minimum of 100 μA for satisfactory diode operation.

Noninverting Schmitt Trigger:

A noninverting Schmitt Trigger Circuit Diagram is shown in Fig. 14-37. This circuit looks like an inverting amplifier, but note that (unlike an inverting amplifier) the inverting input is grounded and the noninverting input is connected to the junction of R₁ and R₂. The waveforms in Fig. 14-37 show that V_o switches rapidly from -V_o(sat) to +V_o(sat) when V_i arrives at the UTP, and that V_o switches back to -V_o(sat) when V_i falls to the LTP.

Consider the situation when V_i = UTP and V_o has just switched to +V_o(sat). The voltage at the junction of R₁ and R₂ is pulled up far above the ground level voltage at the op-amp inverting input terminal. So, the positive voltage at the noninverting input keeps the output at its positive saturation level. To switch the output to -V_o(sat), the voltage at the junction of R₁ and R₂ must be pulled down to the (ground level) voltage at the inverting input terminal. This occurs when V_i is at the LTP, [see Fig. 14-38(a)]. So, switching occurs when one end of R₁ (the right end) is at ground, and the other (left) end is at V_i = LTP; that is, when V_R1 = LTP. Similarly, switching in the other direction occurs when V_R1 = UTP, [Fig. 14- 38(b)]. So, at the trigger points,

Figure 14-38(a) and (b) show that the output voltage is at one of its saturation levels at the instant of triggering. This means that one end of R₂ is at ground (left end), and the other (right) end is at V_o(sat). So, V_R2= V_o(sat) when triggering occurs.

Design procedure for a noninverting Schmitt Trigger Circuit Diagram is just as simple as for the inverting circuit. Voltage divider current I₂ is again selected much larger than the op-amp input bias current.

Then the resistor values are,

Noninverting Schmitt trigger circuits can be designed for different upper and lower trigger point voltages by the use of diodes, as in the case of the inverting circuit. Figure 14-39(a) and (b) shows two possible circuits. The diode forward voltage drop (V_F) must be included in the UTP and LTP calculations.